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published: Feb-27-2015, updated Nov-11-2021

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NO SMOOTHING is applied to the shown plots. Most measurement sites have some smoothing applied which ‘irons flat’ sharp peaks and ‘wiggles’. I do not use smoothing because some info about sound quality is lost when plots are smoothed.

Aside from a small correction of the microphone itself also some correction in the lowest frequencies is applied to the plots to compensate for the perceived loss of bass when using headphones. This is described HERE in more detail.
A ‘horizontal‘ frequency response curve on the shown frequency response plots on this website thus indicates a perceived ‘flat’ tonal signature.

ALL measurements are made with a good SEAL on a flatbed measurement rig.
shape of your head, bone structure, pad size, pad ‘softness,  (compliance), hair or no hair and or wearing glasses may (drastically) change the frequency response of some headphones, so… your personal experience may differ substantially from these plots.

Frequency response (tonal balance) is the most sound-determining aspect of headphones. A horizontal line shows audible neutral response in the plots on this website. Deviations in different severities at different frequency bands have an effect on the sound character.
The bigger the deviation the stronger the effect.

Below an aid to help determining the sound character of headphones with relation to the frequency response.


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AKG K612


The AKG K612 is the successor of the K601.
The K612 looks really nice with its colour accents. The picture above really doesn’t do it justice. It has the same typical construction the other headphones above show. The K612 is made of more luxurious materials and finish.
I expect the elastic bands to fail in a decade or so, just like the headphones above.
Nice leather and metal accents.  The pads are velours and very comfortable. Seems to be similar material as used on the K240 velours pads.
The cable is not too thick and not too thin and supple. Low in microphonics which is a good thing.
It is a very comfortable headphone with enough clamping force and  enough space for the ears. The pad’s inner diameter is 80mm, the height is about 20mm.
A difference it shows compared to the models above is that this headphone can be driven louder.


Type: Over-ear, open
Usage: Home, Studio
Isolation: low (open headphone)
Driver type: dynamic
Pads: replaceable, velour
Pad dimensions: Ø 60mm , depth: 22mm
Collapsible: no
Headphone connector: fixed
Cable entry: left sided.
Cable: 3m with 3.5mm TRS + screw-on 6.35mm adapter
Driver size: ø 40mm
Nom. power rating: 0.2W (200mW)
Max. voltage: 4.9V
Max. current: 40mA
Max. S.P.L.: 115dB
Impedance: 120Ω
Efficiency: 92dB @ 1mW
Sensitivity: 101dB @ 1V
Weight: 238 g. (without cable)
Colour: black with grey accents
Clamping force: medium/low
Accessories: 6.3mm screw-on adapter


Below the frequency response of the K612.FR K612

What’s obvious is the resemblance above 5kHz to the K400/500 above. The K612 has a similar ‘bump’ around 2.5kHz followed by a small dip. The K612 doesn’t show the typical roll-off below 100Hz which the 400/500 series shows. In fact I would say it is rather flat.
There is a small (1.5dB) level matching difference between Left and Right driver between 20Hz and 1kHz but did not really detect it at all. It is there non-the-less and seems to be a driver variation difference. I re-measured it a few times, just to make sure.

Below the phase response of the K612. Above 6kHz the phase shifts are steeper and thus potentially more audible.
The resonances will be smaller when a pinna is present so may be smaller in reality.

phase K612

Below the Group Delay for the K612.GD K612
Only the pad bounce (between 60Hz and 100Hz) is obvious. There are some small issues at 300Hz and 600Hz but are not alarming.

Below the CSD for the K612. The ‘grass’ between 1kHz and 4kHZ  shown in the GD plot above seems caused by resonances.

What’s also obvious is that the 3 peaks (7kHz upwards) are resonances but quite narrow ones.  The treble isn’t really ‘silky’ most likely because of this, and lacks ‘resolution’. The peaks (being elevated above the rest) also give a sense of being a highly detailed headphone where in reality it is not. It is ‘fake’ detail though which becomes obvious when comparing it to ‘better’ headphones. Still… by itself a nice sounding headphone but just below flagship performance in the treble area only.

Below the spectrum plot of the K612.spectr K612
This too shows some ringing and the ‘grass’ is also visible here. Lows are well damped and sound clear and ‘tight’.

Below the step response of the K612.step K612
The ‘sharp’ and detailed but otherwise neutral tonal character is visible. Also the resonance are visible. Even though the horizontal part is somewhat sloping the bass extension is very good. Just not at the desired level.

Below the distortion plot for the K612.dist K612 R
The same plot is shown below but with the more commonly used percentage scale.dist K612 R percent
Distortion is pleasantly low and reaches the ‘measurement floor’ of the test fixture.

The impedance of the K612 is 120 Ω which is a value nicely in-between the more common 32 and 300 Ohm. It can be driven well by portable players directly and C’Moy alike amplifiers as it is fairly efficient and doesn’t need much voltage nor much current which makes it a headphone that is very easy to drive.
The K612 being 120 Ω doesn’t mean it should be driven from a 120 Ω amplifier but it does change its sonic signature slightly on a higher output resistance. In my opinion slightly for the better. The output level drops about 6dB when a 120Ω adapter is used so it won’t play loud enough on portable players any more though.  An amplifier with a 100Ω to 120Ω output R will bump up the sound quality.
The plot below (right channel only) shows how the sound changes when it is fed from a 0 Ω amplifier and a 120 Ω output R amplifier.
Level differences have been removed in this plots to show what the actual change will be.
The lower bass is raised slightly by around 2dB. Also the treble increases and the K612 thus become slightly more U-shaped… just slightly as if one adjusts the bass and treble knobs on your old stereo receiver by about one ‘notch’.

k612 0 ohm vs 120 ohmThe increased upper treble (clarity is not affected, it resides lower in the frequency band) can be lowered quite simply.


I have had fairly good results with damping the treble on some headphones using felt in front of the driver and it appears as though this headphone reacts well to this treatment as well.
Below the frequency response of the K612 with 3 mm felt in front of the driver. The treble spikes are lowered considerably.
As can be seen the leftright differences are still there. I slightly changed the L-R ‘balance’ so the average levels are closer to each other.

k612 felt 3mm

Below a plot showing the differences the felt makes between a stock K612 and with 3mm thick wool-felt in front of the driver.
Right channel only. The felt only lowers the peaks in amplitude and the rest remains exactly the same.

k612 stock vs felt

Below the CSD of the ‘felt treated’ K612. The 2-3kHz resonance is much smaller and resonance around 12kHz is almost completely removed.
The felt doesn’t seem to do that much above 15kHz though. Left, Right

k612 felt csd

The ‘sharp’ treble signature is completely gone and the headphone now has a warm character. I find it more pleasant, but the K612 still shows a ‘low quality’ treble which isn’t masked any more by it being elevated  and sounds a bit ‘coarse’ as in not smooth.
I do think it sounds better this way as I don’t like piercing treble that much these days.
As 3mm felt removed a bit too much of the ‘detail’ I played around with thinner felt (1.5mm) and even toilet paper.The effect you get when using 1.5mm thick felt is right between the red and the green line in the ‘difference plot’.
Some of the ‘harshness’ in the treble is lowered and you don’t get the sense of ‘hyper detail’ any more.  It still has retained the original character but the ‘piercing’ is gone. It is slightly more obvious that the treble isn’t really that  resolving and smooth though but more pleasant to me than when in original form.

The modifications are very easy to do. Simply slightly rotate the pads counter clockwise and the pads come off. On the driver you will notice a foam ring (on the left, in the picture below). Remove that foam ring and cut a piece felt to about the same size as the ring.
K612 modReplace the foam with the felt and put the pads back on. Observe how the holes in the pad’s ring have to align with small tabs on the drivers baffle. Push the pads on the baffle (align the holes and tabs) and rotate the pads a few degrees clockwise while pushing down.

K612 pad modifiedExperiment with felt thickness, or layers of toilet paper, if you are not entirely pleased with the result.
This modification can be undone easily if you do not like what it does to the sound.
It will work similar on K701 and all its iterations as well.

passive inline filter

Another way of reducing the treble peak is by using a passive in-line filter in the headphone cable. The effect of this filter is described in the K702 page but the schematic differs.
The schematic for the K612 filter is shown below, which can also be used with the K601.K612 filter schematic


There is also a filter module available for the Kameleon amplifier.
Below the differences for a stock K612 (K601) and when using the Kameleon

k612 on bass boost

It still sounds like the K612 but it doesn’t ‘lack’ in the bottom and the treble is slightly lower.
With 1.5mm felt it can be lowered a bit more but loses the typical ‘analytical’ nature a bit.


The K612 is probably one of the underexposed headphones in the AKG line. The K612 is a good sounding headphone that may well best the K7** series. Comfort is quite good.
It isn’t a very sensitive headphone so needs a decent amp. Together with some EQ in the lows and an amplifier that can supply enough voltage (it doesn’t require a lot of power) the K612 sounds impressive.
One can expect the elastic bands for regulating the headband height to loose it’s tension after a few years.
Not easy, but possible, to replace.

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